Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline.
A recent development in disk file servo control systems, as described in assignee's U.S. Pat. No. 4,679,103, is a digital servo control system which, as part of the computation of the control signal to the actuator, makes use of a state estimator algorithm to estimate the position and velocity of the head. In this type of servo control system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES and the actuator input current, and computes, through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal and amplified to provide a new actuator input current.
The method of estimating the state of the physical plant to be controlled in a digital control system requires the use of estimator "constants", the derivation of which is described in Digital Control of Dynamic Systems, Franklin and Powell, Addison-Wesley Publishing Co. (1983), chapter 6, pages 131-139. In the case of a disk film, these estimator constants are dependent upon the values of certain physical parameters of the disk file, such as the mass of the coil and head/arm assembly, the actuator force constant (the force applied to the coil per unit of input current), the gain of the VCM power amplifier, the PES gain and the time between PES samples (the PES sampling time).
The estimator constants are generally referred to as "constants" because the values of the physical parameters from which they are determined are generally invariable. However, the PES sampling time, which is the time between receipt of the PES samples by the microprocessor and thus the time between the beginning of consecutive control signal computations by the microprocessor, is a function of the speed of the drive motor which rotates the disks. Typically, disk file drive motors have a specified speed tolerance, e.g. plus/minus 3%. A variation in drive motor speed translates into a variation in the rate at which the head receives servo position information from the disk. Thus if a constant value of PES sampling time, corresponding to the nominal drive motor speed, is used in the estimator constants, the control signal generated by the microprocessor will be in error whenever the drive motor speed varies from its nominal speed. This control signal error will cause the head to undershoot or overshoot the target track when the head is moved between tracks, which could result in an unacceptable delay in the arrival of the head to the target track centerline, or in a seek error.